Well........depends what will be on the sites. What I mean is that if you are going to have basically static HTML pages, and very little or no Dynamic stuff, such as PHP or mySQL, or stuff like that.
I have a P-150 Non MMX, and 64MB of RAM, and I am running 3 sites that have static pages, the sites are accessed quite a bit. One is an internal network IMAP server for e-mails (that has a fair bit of dynamic stuff), and the other 2 are plain jane homepages for my family members.
The IMAP server is hit a lot, At least 2000 views a day, and the 2 homepages are hit maybe 50 a day or 100 a day for the both of them.
There are times when the hard disk chugs, but honestly.......it's not really that bad.
Oh, I forgot to tell you that I am running linux.
So bottom line is......
Get at least an AMD K6-2 450 and 128MB of RAM and any 7200RPM hard disk, and I am sure that it will have more than enough power for your use.